Advanced metal-oxide-semiconductor (MOS) devices require very thin gate dielectric layers that have a thickness on the order of 30 angstroms or less, and that exhibit low leakage currents. Recently high k dielectric materials have been investigated as an alternative to silicon dioxide for use as gate dielectric layers in MOS devices. The high k materials have a higher dielectric constant k than silicon dioxide. This feature enables the further scaling of MOS transistors to smaller dimensions. Equivalent Oxide Thickness (EOT) is a number used to compare performance of MOS transistors having a high-k dielectric layer with the performance of MOS transistors having silicon dioxide gate dielectric layer. The EOT is the thickness of a silicon dioxide gate oxide needed to obtain the same gate capacitance as that obtained with a dielectric having a higher dielectric constant k. For example an EOT of 1 nm would result from the use a 10 nm thick dielectric with a k of 39 (the k of SiO2 is 3.9).
While high-k dielectric materials offer better capacitance than silicone dioxide, these materials are not compatible with the polycrystalline silicon commonly used as a gate electrode material. The electronic conduction band structure of polycrystalline silicon does not match well with high k materials. Accordingly, a metal gate structure is required in MOS devices having a high k gate dielectric in order to fully realize the potential performance improvement. Fabricating the required metal gate structure is expensive and requires advanced metal deposition technology.
In view of the difficulty in incorporating high k dielectric materials in MOS devices, silicon dioxide continues to be used as MOS devices are scaled to smaller geometries. As the EOT of the silicon oxide layer falls below about 20 angstroms, it becomes mandatory to reduce the oxide leakage current. One method of reducing leakage is to include nitrogen in the silicon oxide layer, in which a nitridation process is used to form a silicon oxynitride (SiON) layer. SiON exhibits a relatively high dielectric constant because of nitrogen incorporation and offers reduced leakage currents at an equivalent EOT.
Decoupled-plasma nitridation (DPN) is an emerging new technology for incorporating ultra-high concentrations of nitrogen at the top surface layer of an ultra-thin gate oxide. This process uses inductive coupling for plasma formation. Radio frequency (RF) power is transferred to the plasma via an RF magnetic field, which, in turn, generates an ionizing electric field. Inductive coupling is much more efficient for plasma production than a direct electrode plasma since energy is not dissipated in driving ions into the electrode surfaces.
Introducing nitrogen into a silicon oxide gate layer by DPN improves the nitrogen uniformity and oxide leakage current considerably. While the DPN process advantageously operates at low temperatures, a post nitridation anneal (PNA) is required to remove low-temperature induced defects in the SiON layer. Hence, post-DPN annealing at a relatively high temperature is required to eliminate the defects and improve the oxide integrity.
Several prior art references disclose plasma nitridation processes. For example, U.S. Pat. No. 6,140,187 to DeBusk et al. discloses a remote plasma nitridation process for a gate oxide. A He/Ar plasma is used followed by an anneal in oxygen at about 800° C. In U.S. Pat. No. 5,861,329 to Yeh, a plasma process for forming a barrier layer is discloses. Gases used in the process include nitrogen, ammonia, nitrogen oxide, and nitrogen/oxygen mixtures. U.S. Pat. No. 6,225,169 to Chew discloses a RTN process in which the nitrided layer is formed on the sidewalls of the gate structure, while U.S. Pat. No. 6,162,717 to Yeh et al. discloses a high density plasma process in which the gate dielectric becomes sandwiched between two layers of silicon nitride.
U.S. Pat. No. 7,176,094 to Zhong et al., and assigned to the instant assignee, discloses the formation of a nitrided silicon oxide layer by DPN and PNA in which the PNA is carried out in a 1:4 oxygen-nitrogen mixture. The nitridation process produces a gate oxide layer that is resistant to boron penetration.
Research has shown that increased nitrogen concentrations improve the performance of SiON material as a gate dielectric material. As the total nitrogen concentration increase, however, excessive amounts of nitrogen appear at the oxide/silicon interface, which degrades the SiON performance. The nitrogen build-up at the interface leads to a trade off between fabricating SiON layers having either low gate leakage or low EOT. When one is reduced, the other increases. While substantial work in the development of nitrided oxide layers for use as a gate dielectric in MOS devices has shown this material to be an effective alternative to silicon dioxide, a need existed to improve the total nitrogen concentration and nitrogen profile in SiON gate dielectric layers